1. Field of the Invention
The present invention relates to an electrode material for a rechargeable lithium battery in which oxidation-reduction reaction of lithium is used, said electrode material comprising a fine powder of a silicon-based material whose principal component is silicon and which has an average particle size in a range of more than 0.1 μm to less than 0.5 μm which means that when said average particle size is represented by R, 0.1 μm≦R<0.5 μm, where the silicon-based material fine powder preferably contains oxygen element in a small amount such that at least particles constituting the silicon-based material fine powder are covered by a thin oxide film with respect their surfaces. The present invention also relates to an electrode structural body comprising said electrode material and a rechargeable lithium battery whose anode comprising said electrode structural body. The present invention includes a process for the production of said electrode structural body and a process for the production of said rechargeable lithium battery.
2. Prior Art
In recent years, the global warming of the earth because of the so-called greenhouse effect due to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is being converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is being exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting to newly establish a thermal electric power plant. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Now, for electric vehicles which do not exhaust any air polluting substances such as CO2, NOx, hydrocarbons and the like, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein. Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, portable type MD players, video cameras, digital cameras and cellular phones.
As such miniature, lightweight and high performance rechargeable battery, there have proposed lithium ion batteries of the so-called rocking chair type in that a carbonous material such as graphite capable of intercalating lithium ion at intercalation sites of its six-membered network plane provided by carbon atoms in the battery reaction upon charging is used as an anode material and a lithium intercalation compound capable of deintercalating said lithium ion from the intercalation in the battery reaction upon charging is used as a cathode material. Some of these lithium ion batteries have been put to practical use.
However, for any of these lithium ion batteries whose anode comprising the carbonous material (the graphite), the theoretical amount of lithium which can be intercalated by the anode is only an amount of ⅙ per carbon atom at the most. Because of this, in such lithium ion battery, when the amount of lithium intercalated by the anode comprising the carbonous material (the graphite) is made to be greater than the theoretical amount upon charging or when charging is performed under condition of high electric current density, there will be an unavoidable problem such that lithium is deposited in a dendritic state (that is, in the form of a dendrite) on the surface of the anode. This will result in causing internal-shorts between the anode and the cathode upon repeating the charge-and-discharge cycle. Therefore, it is difficult for the lithium ion battery whose anode comprising the carbonous material (the graphite) to achieve a high capacity.
Now, rechargeable lithium batteries in which a metallic lithium is used as the anode have been proposed and they have attracted public attention in a viewpoint that they exhibit a high energy density. However, such rechargeable battery has not been put to practical use because the charge-and-discharge cycle life is extremely short. A main reason why the charge-and-discharge cycle life is extremely short has been generally considered as will be described in the following. The metallic lithium as the anode reacts with impurities such as moisture or an organic solvent contained in an electrolyte solution to form an insulating film or/and the metallic lithium as the anode has an irregular surface with portions to which electric field is converged, and these factors lead to generating a dendrite of lithium upon repeating the charge-and-discharge cycle, resulting in internal-shorts between the anode and cathode. As a result, the charge-and-discharge cycle life of the rechargeable battery is extremely shortened.
In order to eliminate the problems of the rechargeable battery in which the metallic lithium is used as the anode, specifically, in order to suppress the progress of the reaction between the metallic lithium of the anode and the moisture or the organic solvent contained in the electrolyte solution, there has been proposed a method in that a lithium alloy such as a lithium-aluminum alloy is used as the anode. However, this method is not widely applicable in practice for the following reasons. The lithium alloy is hard and is difficult to wind into a spiral form and therefore, it is difficult to produce a spiral-wound cylindrical rechargeable battery. Accordingly, it is difficult to attain a rechargeable battery having a sufficiently long charge-and-discharge cycle life. It is also difficult to attain a rechargeable battery having a sufficient energy density similar to that of a primary battery in which a metallic lithium is used as the anode.
There are various proposals in order to such problems as described in the above. Particularly, U.S. Pat. No. 5,795,679 discloses an anode for a rechargeable lithium battery, comprising a powder of an alloy comprising a metal element such as Ni or Cu and Sn or the like.
U.S. Pat. No. 6,051,340 discloses an anode for a rechargeable lithium battery, comprising a collector comprising a first metal such as Ni or Cu which is incapable of being alloyed with lithium which is generated upon charging and an electrode layer on said collector, comprising said first metal and a second metal such as Si or Sn which is capable of being alloyed with lithium which is generated upon charging.
U.S. Pat. No. 6,432,585 B1 discloses an anode for a rechargeable lithium battery, having an electrode material layer containing 35 wt. % or more of a grained host matrix material comprising Si or Sn and which has an average particle size in a range of from 0.5 to 60 μm.
Japanese Laid-open Patent Publication No. Hei. 11(1999)-283627 discloses an anode for a rechargeable lithium battery, comprising an amorphous phase-bearing metallic material of Si or Sn.
Japanese Laid-open Patent Publication No. 2000-311681 discloses an anode for a rechargeable lithium battery, comprising a particulate of an amorphous Sn-A (transition metal element) with a substantially non-stoichiometric composition.
International Laid-open Publication WO 00/17949 discloses an anode for a rechargeable lithium battery, comprising a particulate of an amorphous Si-A (transition metal element) with a substantially non-stoichiometric composition.
Japanese Laid-open Patent Publication No. 2000-215887 discloses an anode material for a rechargeable lithium battery, comprising a particulate comprising a number of separate particles of a metal or a semimetal capable of being alloyed with lithium, said particles having a surface covered by a carbon layer, specifically, silicon particles whose surfaces are covered by a carbon layer, wherein the carbon layer is formed by a method of subjecting a carbon supply source such as benzene or the like to a chemical evaporation treatment through thermal decomposition.
By the way, as one of the compounds comprising Li and Si, there is known a Li4.4Si alloy compound. For this compound, when a maximum theoretical electricity storable capacity is calculated by presuming that the Li can be entirely released in the electrochemical reaction, the maximum theoretical electricity storable capacity becomes to be 4200 mAh/g. In view of this, it has been expected that if a silicon-based material could be put to use as an anode material for a rechargeable lithium battery, it will be possible to attain a rechargeable lithium battery having a high capacity. And various studies have being conducted in order to achieve an anode comprising such silicon-based material which enables to produce a rechargeable lithium battery having a high capacity.
However, any of the anodes described in the above-mentioned documents is difficult to attain an electrode performance in that lithium can be inserted or leased at a high electricity quantity which is exceeding 1000 mAh/g. Thus, there is an increased demand for providing a high capacity anode for a rechargeable lithium battery, having an electrode performance in that lithium can be inserted or leased at a high electricity quantity which is exceeding 1000 mAh/g.